1. Field of the Invention
This invention relates generally to communication systems. More particularly, this invention is related to transceivers in communication systems.
2. Description of the Prior Art
For a high-speed IO buffer, there are I-V curve (or impedance) characteristic requirements for both a driving mode and a receiving mode. FIG. 1 shows a conventional IO buffer 100. The IO buffer 100 includes a driver 102 and a receiver 104. A control unit 112 is used to assert or negate transistor Q1, Q2, Q3, and Q4 in response to the Data and OE signals.
In the driving mode, OE (data output enable) is asserted. Transistors Q3 and Q4 are off. When a logic state 1 (Data) is to be outputted though node 106, transistor Q1 is on and transistor Q2 is turned off to pull high the node 106. When a logic state 0 (Data) is to be outputted through node 106, transistor Q1 is turned off and Q2 is turned on to pull low the node 106. In the receiving mode, OE (data output enable) is negated. Both transistors Q1 and Q2 are off and transistors Q3 and Q4 are turned on.
FIG. 2A shows an I-V diagram of transistor Q2 for a driving mode. FIG. 2B shows an I-V diagram of transistor Q1 for a driving mode. There may be different requirements for different applications. For example, in a driving mode for a specific application, the I-V curve 202 of transistor Q2 is required to be designed between a maximum curve 205 and a minimum curve 206. Similarly, the I-V curve 204 of transistor Q1 is required to be designed between a maximum curve 207 and a minimum curve 208.
FIG. 2C shows an I-V diagram of transistor Q3 for a receiving mode. FIG. 2D shows an I-V diagram of transistor Q4 for a receiving mode. For the receiving mode, the impedance of the receiver 104 is required to match that of the transmission line 114 coupled to node 106. The I-V curve 211 of transistor Q3 is required to be linear and between a maximum curve 210 and minimum curve 212. The I-V curve 214 of transistor Q4 is required to be linear and between a maximum curve 214 and minimum curve 216.
Because the driving mode and the receiving mode have different requirements, they are conventionally designed separately in an IO buffer. The area is thus larger and the IO buffer is less flexible for different applications where different impedance and linearity (constant impedance) requirements are needed. Therefore, there is a need for a new IO buffer that can reduce area and increase flexibility.